1. Field of the Invention
The present invention relates generally to a time information acquisition apparatus which receives a standard time radio wave to acquire time information thereof, and a radio wave timepiece on which the time information acquisition apparatus is mounted.
2. Description of the Related Art
In recent years, for example, in Japan, Germany, England, and Switzerland, transmitting stations transmit a standard time radio wave of a low frequency. For example, transmitting stations in Fukushima and Saga prefectures in Japan transmit amplitude-modulated standard time radio waves of 40 kHz and 60 kHz. The standard time radio wave includes a code string forming a time code indicating the date and time and is transmitted every 60 seconds. That is, the period of the time code is 60 seconds.
A clock (radio wave timepiece) that receives the standard time radio wave, extracts the time code from the received standard time radio wave, and corrects the time has been put to practical use. A receiver of the radio wave timepiece includes a band-pass filter (BPF) that receives the standard time radio wave through an antenna and extracts only the standard time radio wave signal, a demodulator that demodulates an amplitude-modulated standard time radio wave signal using, for example, envelope detection, and a processor that reads a time code included in the signal demodulated by the demodulator.
The processor in the prior art performs synchronization with the rising edge of the demodulated signal and then performs binarization with a predetermined sampling period to acquire time code output (TCO) data having a unit time length (one second), which is a binary bit string. The processor measures the pulse width (that is, the time of a bit 1 or the time of a bit 0) of the TCO data, determines whether each code is a binary 1 code, a binary 0 code, or a position marker code P based on the measured pulse width, and acquires time information based on the determined code string.
The processing circuit according to the prior art performs processes, such as a second synchronization process, a minute synchronization process, a process of acquiring a code, and a process of determining matching, during the period from the start of the reception of the standard time radio wave to the acquisition of the time information. When each of the processes is not appropriately terminated, the processing circuit needs to start the processes from the beginning. Therefore, in some cases, the processing circuit needs to start the processes from the beginning several times due to the influence of noise included in the signal. Under such instances, it takes a very long time to acquire time information.
The second synchronization detects the rising edge of a code at an interval of one second among the codes indicated by the TCO data. It is possible to detect a portion in which a position marker P0 arranged at the end of a frame and a marker M arranged at the head of the frame are continuously arranged by repeatedly performing the second synchronization. The portion in which the markers are continuously arranged appears at an interval of one minute (60 seconds). Within the TCO data, the marker M shows the position of the head frame data. The detection of the position of the marker is referred to as minute synchronization. The head of the frame is recognized by the minute synchronization. Therefore, after code acquisition starts to acquire one frame of data, a parity bit is checked to determine whether the data has an improper value (the date and time have improper values) (matching determination). For example, since the minute synchronization is for detecting the head of the frame, 60 seconds are required in some cases. Of course, multiples of 60 seconds are required to detect the heads of several frames.
In Jpn. Pat. Appln. KOKAI Publication No. 2005-249632 (corresponding to US 2005/0195690 A1), the demodulated signal is binarized at a predetermined sampling interval (50 ms) to obtain TCO data and a list of data groups (20 samples) in the form of binary bit strings is obtained every one second.
The apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2005-249632 compares the bit string with each of the templates of a binary bit string indicating a position marker code P, a binary bit string indicating a code 1, and a binary bit string indicating a code 0, calculates a correlation therebetween, and determines to which of the codes P, 1, and 0 the bit string corresponds, based on the correlation.
In the technique disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2005-249632, the TCO data, which is a binary bit string, is acquired and matched with the template. When the field intensity is weak or a large amount of noise is mixed with the demodulated signal, many errors are included in the acquired TCO data. Therefore, it is necessary to provide a filter which removes noise from the demodulated signal or to finely adjust the threshold of an AD converter, in order to improve the quality of the TCO data.
JPn. Pat. Appln. KOKAI Publication No. 2009-216544 (corresponding to US 2009/0231963 A1) discloses a technology for generating input waveform data corresponding to one frame (60 seconds), generating predicted waveform data which has the same data length as the input waveform data and is associated with a current time conforming to a time (a base time) based on an internal clock, comparing a sample value of the input waveform data with a corresponding sample value of the predicted waveform data, and detecting the number of errors. According to the technology in JPn. Pat. Appln. KOKAI No. 2009-216544 (corresponding to US 2009/0231963 A1), the predicted waveform data is shifted by one bit (a sample value at the end of the data becomes a sample value at the head of the same), and comparison between the sample value of the input waveform data and a new corresponding sample value of the shifted predicted waveform data is repeated. The processing is repeated for 60 times, predicted waveform data having the smallest number of errors is found based on the numbers of errors in the respective pieces of predicted waveform data, and an error of the base time is acquired based on a shift number of the found predicted waveform data.
The technology in JPn. Pat. Appln. KOKAI No. 2009-216544 (corresponding to US 2009/0231963 A1) requires input waveform data corresponding to 60 seconds. Additionally, it is required to generate 60 types of predicted waveform data by a shifting operation and to compare the sample value of the input waveform data with the sample value of the predicted waveform data. Therefore, there is a problem that the acquisition of the input waveform data and the comparison of the sample values require a processing time. Further, since an electric wave receiving status is not necessarily constant, reducing a reception time for the standard time radio wave is desired to acquire the input waveform data.